The State Quarters program is noteworthy for many reasons, but perhaps the most interesting to U.S. coin collectors is the number of error coins that have appeared in the State Quarter series. The coin featured here is an example of the struck through grease or filled die error type. This error is particularly interesting because it changed a word to another word. In fact, the error makes it sort of funny.

Looking at the image, you can see that the first "T" in "TRUST" seems to be not been punched/die hard enough, rendering a legend that says, "In God We Rust." If you look real closely, though, you can see a faint outline of the T. 

Why is the "T" Missing?

Like all mechanical machines, a lubricant is needed in the coin press to prevent the metal parts from causing friction to each other. In this case, some lubricating grease escaped from the machinery onto the surface of the coin die, eventually plugging up the letter T in TRUST. With the T being plugged, when the coin dies struck the planchet, the T wasn't fully transferred to the surface of the coin. This type of error is called a struck through grease filled die (or just "filled die") error.

Sometimes, the grease can be combined with dirt and other impurities to form a hard compound. When it gets stuck between the coin die and the planchet, it will become even harder under the pressure of the striking chamber. In this case, the filled coin die can be used to strike thousands of coins before it falls out or a press operator cleans the coin die.

As the coin die continues to strike blanks and coins, the grease continues to be compressed and becomes thinner and thinner. Eventually, the grease may actually fall out of the recessed area of the dies. At this point, the die will continue to strike normal coins with the entire word.